In applications such as inspection, control and automation of complex processes normally utilized in metrology, it is usual to make use of a number of measuring instruments such as instruments for measuring temperature, pressure, flow rate or level at the same time. A measuring instrument generally consists of a measuring sensor and an evaluation instrument or switching instrument arranged at a distance therefrom. The instruments have to be switched on individually, that is, they have to be supplied with power and, if required, signal lines have to be connected to the instruments and lead away from them.
For this reason, in industrial plants switchboards are usually installed accommodating a plurality of instruments which are spatially closely adjacent to one another. These instruments are set on supports provided therefor and are fastened detachably onto their respective supports by means of snap locks or screw plugs. The supports are fitted to a wall, for instance, or are snap-locked to a rail, particularly a top hat rail.
For reasons of safety and functionality of such plants it is particularly important that assignment of the individual instruments to the supports provided for them is carried out correctly. Any faulty setting and accordingly incorrect wiring of the corresponding instruments can have fatal consequences.
DE-A 39 33 703, corresponding to EP-A 422 568, discloses an instrument system having at least one instrument disposed in a housing, wherein the housing is mounted detachably on a top hat rail. Embedded in the top hat rail is a plastic profile in which electrical wiring is arranged to which the instrument is connected when locked on to the top hat rail.
Attached to a rear wall of the instruments facing the plastic profile is a guide pin which engages in a corresponding recess in the plastic profile when the instrument is locked on. The guide pin can also be used for coding. Coding pins can also be set into the plastic profile, which engage on the underside of the instrument when it is set in corresponding recesses.